The present invention relates to safety equipment for installations containing a medium under pressure.
Safety equipment for installations containing a medium under pressure is widely used in technology. It is used, for example, in high pressure technology, in container technology, in chemical technology and also in nuclear reactor technology. In nuclear reactor technology, such equipment is used especially for gas-cooled reactors, high temperature reactors, liquid cooled reactors and also for reactor pressure lines and reactor pressure vessels.
This safety equipment is intended to reduce and to limit to a predetermined area any immediate effects of damage which can occur due to the bursting of pressure installations bulkheads.
Safety equipment of the above-described type, intended to guarantee that the fluid stream occasioned by the escape of the pressurized medium from the installation is limited to a minimum, has been proposed in different embodiments. Thus, for example, safety equipment used in the live steam lines of boiling water reactors and designated as "flow limiter" is known. This device is developed as a nozzle, especially as a veturi, and is to be disposed in a steam boss of the reactor pressure vessel. In this way the rate of pressure reduction in the neclear reactor due to line ruptures outside of the reactor is reduced. In order to further reduce the rate of the pressure reduction in the reactor, it has been additionally proposed to provide rapidly closing reflex flaps in the lines. The disposition of such flow limiters have proven successful in prolonging the duration of the pressure drop, during a case of damage, so that a pressure drop of from 70 atmospheres to 15 atmospheres required approximately 150 seconds. As a result, damage to the fuel elements was avoided. However, when such flow limiters are used with boiling water reactors, the resulting cross sectional reduction also limits the flow of the cooling medium from the reactor pressure vessel. The consequence of this is that it is impossible to increase the effect of a flow limiter by reducing the flow cross section to be limited. The flow cross section must be at least large enough so that under the condition of the highest possible velocity, i.e. the velocity of sound, the mass flow rate of the normal cooling medium can still pass through the limited flow cross section. Furthermore, when flow limiters are used, the necessary deceleration of the pressurized medium which is accelerated up to the limiting flow cross section, brings with it unavoidable losses. A reduction of the limiting flow cross section, which is desirable in order to increase the effectiveness of the safety equipment, is therefore not possible at all or not in the desired measure. If this safety equipment is used during a disturbance, then the stream passing through the flow limiter can be reduced to the value of the normal stream passing through the pressure line only if considerable energy losses are accepted. It is not possible to reduce the value of this steam below the normal value.
In other know safety equipment, which is used especially in pipe lines, it is provided that in a particular predetermined section of the pipe line two coaxially disposed pipes are provided whose walls are dimensioned for the full pressure difference. The space between the inner and outer pipe is filled with a medium that is under such pressure that the inner pipe is pressure relieved in normal operation, whereas the outer pipe carries the full pressure. In this way, losses are eliminated when rupture of the outer pipe occurs because the full pressure is taken over the inner pipe. The design is such that the pressure is maintained for a predetermined period of time. What is disadvantageous in the use of this safety equipment is that it is necessary to provide connections between the inner and outer pipes. Because of this, tensile stresses are created between the two pipes. These stresses are generated during manufacture of they occur because of the forces developed during installation. They can also be caused through changes which occur in the outer mounting conditions, for example, by long term creep or construction settling, or they can be caused by temperature fluctuations. Added to this is the fact that forces, for example, pressure forces, exerted on one line, can produce forces in the other line. If the pressure control fails, the pressure pad which loads the inner line can produce stresses between the two lines.
If this known safety equipment is used in hot gas lines, then further limitations accrue from the generated internal stresses. Furthermore, even in the most general case, when the inner pipe is designed with due consideration for all the possible loads that can occur in an emergency, further difficulties accrue because of the devices and compensators that must be provided. Required checks or tests of the inner line are furthermore possible only with considerable effort, if at all.